The term portable “hand-held” wireless device was, at one time, reserved for small, personal digital assistants (PDAs) or cell phones. However, these portable devices have expanded well beyond simple telephonic communications and now support a broader array of applications. Cameras, music players and Internet browsers are commonplace in portable devices.
Another technology that is rapidly making its way into portable devices such as cell phones is digital television. The standard defining digital television in portable devices is the Digital Video Broadcast-Handheld (DVB-H) standard. One of the challenges associated with providing DVB-H to handheld devices is the antenna necessary to receive the broadcast signal.
An antenna, when used to receive signals, converts electromagnetic waves into voltage. The antenna is a conductor placed within an electromagnetic field to induce a voltage that carries the received signal. The antenna is most efficient when the electrical length of the antenna is equal to the wavelength of the signal that is desired to be received. The resonant frequency of the antenna is related to its electrical length, and defines the frequency at which the antenna is tuned when receiving an electromagnetic field. The bandwidth of the antenna is the range of frequencies over which the antenna is effective, generally centered upon the resonant frequency. The resonant frequency may be changed by changing the electrical length of the antenna.
Traditional resonant antennas without any adjustments would be useful in a very small part of the DVB-H spectrum (450-702 MHz). The reception/transmission efficiency of the signals away from an antenna's resonant point may be increased by creating stubs near the feeding point. These stubs produce equal and opposite reflections to the reflections created by the impedance mismatch that exists between the antenna and the input circuitry away from resonance. Many alternative topologies use the tuning these stubs in order to achieve effective wideband operation.
Another known approach involves designing wideband patch antennas. The problem associated with this approach is the thickness requirement to achieve the desired bandwidth as discussed in (“BW˜patch/thickness/lambda”, David R Jackson, formula (44) from IEEE Transactions on Antennas and Propagation, vol. 39, No. 3, March 1991). After widening the bandwidth to more than 10%, the radiation efficiency drops very quickly and for DVB-H the bandwidth needed exceeds 40%. Therefore, the necessary increase in thickness decreases efficiency and the resulting increased volume taken up by the antenna makes them less appealing for small, handheld devices. It would be beneficial to have a small, tunable antenna to address these drawbacks.